An EL device which utilizes the electroluminescence is characterized by a high visual-distinguishability because of its self-emission, excellent impact resistance because of its being in the form of a complete solid device, and the like properties. Therefore, the EL device has been attracting attention for utilization as a light-emitting device in a variety of types of display apparatuses.
The EL device is divided into an inorganic EL device in which an inorganic compound is used as the light-emitting material, and an organic EL device in which an organic compound is used as the light-emitting material. Of these, the organic EL device, which can decrease the applied voltage to a great extent, has been positively studied for practical application as a display device of the next generation.
It is evident that multi-coloring is required for a display device in order to expand the use and application field of the above-mentioned organic EL device, as can be seen from the examples of the cathode-ray tube (CRT) and the liquid crystal display (LCD).
Several methods have heretofore been known as a method for preparing a multi-colored display apparatus by the use of an EL device, including, for example, (1) a method in which EL materials which emit light with three primary colors of red (R), green (G) and blue (B), respectively are each arranged in the form of matrix [refer to Japanese Patent Application Laid-Open Nos. 157487/1982 (Sho-57), 147989/1983 (Sho-58), 214593/1991 (Hei-3), etc.; (2) a method in which an EL material which emits light with white color is combined with a color filter to take out the three primary colors of R,G,B [refer to Japanese Patent Application Laid-Open Nos. 315988/1989 (Hei-1), 273496/1990 (Hei-2), 194895/1991 (Hei-3), etc.; (3) a method in which an EL material which emits light with blue color is combined with a fluorescence conversion membrane to take out the three primary colors of R,G,B [refer to Japanese Patent Application Laid-Open No. 152897/1991 (Hei-3), etc. and the like methods. However, the above-mentioned method (1) suffers the disadvantages that the three kinds of light-emitting materials must be arranged in the form of matrix in high precision and fineness, thereby causing technological difficulty and failure in its production at a low cost, and further that the three kinds of light-emitting materials usually have each a service life frequently different from one another, whereby the chromaticity of the emitted light deviates from the normal value with the lapse of time. On the other hand, the method (2) suffers the drawback that the utilization efficiency of the EL light is lowered, that is, the conversion efficiency is lowered, since part of the output light from the EL device which emits light with white color is taken out with a color filter to utilize the light. For example, when red color is taken out by the use of a color filter from a white El color consisting simply of the three primary colors each having a same intensity, the maximum obtainable conversion efficiency is only 33%. In practical application, however, the conversion efficiency is much lower than 33% taking into consideration the emission spectrum and visibility. The method (3) is made superior to the aforesaid method (2), if the three primary colors of R,G,B are each obtained at a conversion efficiency of at least 33% in the method (3).
There is well known a method in which fluorescence conversion membranes are arranged on an EL device to versatilely vary the color tone of the EL-emitting light color [refer to Japanese Patent Application Laid-Open Nos. 18319/1988 (Sho-63) and 152897/1991 (Hei-3)]. The blue color among R,G,B is emitted from the organic EL device itself, and thus may be utilized as such. In this case if the conversion efficiency is forced to be stated, it is 100%. With regard to green, a conversion efficiency of 80% is obtained by the use of coumarin 153 as is disclosed in Japanese Patent Application Laid-Open No. 152897/1991 (Hei-3)]. Nevertheless there is not yet known so far a method for converting the blue light of an EL device to red light at a conversion efficiency of at least 33%. For example, as is disclosed in Japanese Patent Publication Nos. 32879/1993 (Hei-5) and 33514/1993 (Hei-5), the light-emitting layer of the blue/green light-emitting inorganic EL device in which layer is dispersed rhodamine, that is, a red fluorescent coloring material, emits white light, thus failing to emit objective red light. likewise, white light instead of the objective red light is emitted form the blue/green light-emitting inorganic EL device the outside of which is fitted with the fluorescence conversion membrane composed of rhodamine B [Japanese Utility Model Application Laid-Open No.77299/1988 (Sho-63)], also from the blue/green light-emitting inorganic El device the outside of which is fitted with the fluorescence conversion membrane composed of the pink base fluorescent coloring material (produced by Sinloihi Co. Ltd. under the trade name "FA001") [Japanese patent Application Laid-Open No. 163159/1994 (Hei-6)]. In the case where the blue light-emitting organic EL device is fitted with the fluorescence conversion membrane composed of phenoxazone 9 and a color filter for regulating chromaticity [Japanese Patent Application Laid-Open No. 152897/1991 (Hei-3)], there is obtained a red light having a chromaticity, x of 0.62 and y of 0.33, but the converted light thus obtained is so faint as is visible only in a bright place with an extremely low conversion efficiency.
Japanese Patent Application Laid-Open No. 158091/1990 (Hei-2) describes the fluorescent substance having a main stimulating wavelength in the range of 440 to 560 nm and a main light-emitting wavelength in the range of 510 to 650 nm. However, the above-mentioned fluorescent substance is devoid of an absorption capable of interrupting at least blue color and consequently, can emit white color only.
Further, Japanese Patent Application Laid-Open No. 2205971/1985 (Sho-60) describes the wavelength-conversion fluorescent substance which absorbs the light having a peak wavelength in the range of 460 to 520 nm and emits light having a peak wavelength in the range of 590 to 610 nm. However, the aforesaid wavelength-conversion fluorescent substance fails to employ a fluorescent substance capable of selectively interrupting the blue color having a wavelength in the range of 460 to 520 nm and therefore, the fluorescent substance is incapable of selectively emitting red color.
As described hereinbefore, since the red fluorescence pigment which is typified by rhodamine base fluorescence pigment and phenoxazone base fluorescence pigment is usually devoid of an absorption in the blue color region, the independent use of the aforesaid red fluorescence pigment in a fluorescence-reddening membrane leads to failure to sufficiently interrupt the original blue light and as a result, to selectively obtain the objective red color by reason of the mixing of the blue light with the conversion light of red color. In the case where a color filter for chromaticity regulation is superimposed on the fluorescence-reddening membrane in order to interrupt the original blue light, the red conversion efficiency is inevitably lowered.